A Proton Exchange Membrane ("PEM") fuel cell converts the chemical energy of fuels such as hydrogen and air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (H.sup.+ ions) from the "anode" side of a fuel cell to the "cathode" side of the fuel cell while preventing passage therethrough of the hydrogen and air/oxygen gases. Some artisans consider the acronym "PEM" to represent "Polymer Electrolyte Membrane." The direction, from anode to cathode, of flow of protons serves as the basis for labeling an "anode" side and a "cathode" side of every layer in the fuel cell (and fuel cell assembly or stack, as described below).
An individual fuel cell generally has multiple, transversely extending layers assembled in a longitudinal direction. Furthermore, fluid manifolds extend longitudinally through the periphery of each fuel cell. The fluid manifolds "service" the fluids for each fuel cell, as described below.
As is well-known in the art, the fluid manifolds distribute hydrogen and air/oxygen to, and remove unused hydrogen and air/oxygen as well as product water from, fluid flow plates of the fuel cell. Often, the hydrogen and air/oxygen gases are humidified before entering the manifolds in order to carry water for humidification of the PEM of the fuel cell. Furthermore, the fluid manifolds circulate water for cooling.
Typically, the chemical reactions of a single fuel cell generate a relatively small voltage (e.g., 0.4 to 0.9 volts). In order to provide external usage of desirable voltages such as between 1 and 400 volts, one usually connects a plurality of the fuel cells in series. That is, one assembles the plurality of fuel cells in a "stack."
In a fuel cell assembly or stack, all layers that extend to the periphery of the fuel cells have holes therethrough for alignment and formation of the fluid manifolds. Further, gaskets seal these holes and cooperate with the longitudinal extents of the layers for completion of the fluid manifolds.
Generally, the longitudinally "first" and "last" fluid flow plates, at the respective "anode" and "cathode" ends of the stack of fuel cells, are electrically coupled to "anode" and "cathode" current collector plates, respectively. One can form the current collector plates from, for instance, copper.
Furthermore, the "cathode" current collector plate is coupled to an external load which is in turn coupled to the "anode" current collector plate for completion of a circuit. One makes use of the fuel cell assembly by this connection of the external load within the conduction path between the "cathode" and "anode" current collector plates. For example, the electrical connections can employ wire or cable.
Outward from each current collector plate in a fuel cell assembly usually appear a separate insulating layer followed by an end plate. The end plates typically have holes for receiving tie bolts that hold the fuel cells in compression between the end plates. For example, the end plates ("anode" and "cathode") can be made of aluminum.
Various configurations exist for servicing the fluids involved in the chemical reactions and cooling which desirably occur in the fuel cell assembly. In one known design, the current collector plate, insulating layer, and end plate each have therethrough manifold holes. This configuration presents a risk to the fuel cells, particularly to each PEM, of contamination by metallic ions from the end plate or the current collector plate. Furthermore, the holes in the current collector plate can shunt electrical current.
Another design includes a bypass for servicing fluid to the fuel cells. In particular, the bypass requires a separate mechanical structure for transmission of fluid around the current collector plate, insulating layer, and end plate. By employing additional plates, tubes, and/or seals, this bypass adds complexity and increases material usage.
A number of configurations exist for externally conducting the electrical current of the fuel cell assembly. One design provides an outwardly (e.g., upwardly) extending neck on a current collector plate. This neck provides a contact surface for conduction of the electrical current to a connecting cable or bus bar. An insulation layer followed by an end plate appear outwardly from this current collector plate.
Another design provides insulated connection of an external load through an end plate. In particular, a sleeve insulates a terminal lug as it passes through the end plate and the insulator for connection to the current collector plate. Such a design is disclosed in U.S. Pat. No. 4,719,157 to Tsutsumi et al. (entitled "Fuel Cell Stack Assembly," issued Jan. 12, 1988, and assigned to Sanyo Electric Company).
Thus, a need exists for a new end plate construction that not only provides structural support for a fuel cell assembly, but also facilitates current conduction and fluid service without the complexity, potential contamination hazards, electrical shorting problems, and other drawbacks attendant in prior designs.